DifferenceEngine: Make timestamps data readable

The Minerva skin renders a relative timestamp.
Having this data attribute available will allow the Minerva
skin to render a relative timestamp on this page.

Bug: T117279
Change-Id: Ib66de95bdeca6e9bfa082889f0ed0740dbf8639f

includes/diff/DifferenceEngine.php

@@ -1798,7 +1798,9 @@ class DifferenceEngine extends ContextSource {
 			$header = $this->linkRenderer->makeKnownLink(
 				$title,
 				$header->text(),
-				[],
+				[
+					'data-timestamp' => $revtimestamp,
+				],
 				[ 'oldid' => $rev->getId() ]
 			);
 			$editQuery = [ 'action' => 'edit' ];